Use LLVM 12 (instead of latest/14) to generate Windows bitcode.

We need the LLVM version we use here to generate the bitcode
to not be newer than the version we'll use in the compiler
to read in the bitcode: 13.0.0 (as set in the `llvm-static` repo).

We specify LLVM 12 instead of LLVM 13 here because the latest
available minor or patch version of LLVM 13 may be newer than
the one we have as our limit (13.0.0).

.cirrus.yml

@@ -89,7 +89,7 @@ build_task:
         image: ubuntu:22.04
       environment:
         TRIPLE: x86_64-unknown-windows-msvc
-        DEPS_INSTALL_PRE: apt-get update && apt-get install -y --no-install-recommends apt-transport-https ca-certificates clang cmake curl git llvm make
+        DEPS_INSTALL_PRE: apt-get update && apt-get install -y --no-install-recommends apt-transport-https ca-certificates clang-12 cmake curl git llvm-12 make && ln -s /usr/bin/clang-12 /usr/bin/clang && ln -s /usr/bin/clang++-12 /usr/bin/clang++ && ln -s /usr/bin/llvm-link-12 /usr/bin/llvm-link
         DEPS_INSTALL: curl -L --fail ${XWIN_RELEASE_URL} | tar --strip-components=1 --wildcards -xvzf - '*/xwin'
         # Without this next environment var, apt-get will try to ask us
         # interactive questions, to which we will be unable to respond...
@@ -106,6 +106,9 @@ build_task:
 
   deps_script:
     - sh -xc "${DEPS_INSTALL_PRE:-echo}" && sh -xc "${DEPS_INSTALL:-echo}"
+    - clang --version
+    - clang++ --version
+    - llvm-link --version
 
   xwin_cache:
     folder: /tmp/xwin